If sunlight slowly spoils spices, why are they sold in clear glass jars?

Clear glass appeals to more customers. But you can buy most spices in cardboard packets and transfer to an airtight container once opened. The main thing is to store them in a cool dark place, as sunlight slowly causes the essential oils to disintegrate and lose flavour. While some spices hold their flavour for years, most are best replaced every year - try writing the date of purchase on a sticker beneath the container.